1. Field of the Invention
The present invention relates generally to systems and methods for determining thin-film thickness in the semiconductor process and more particularly to systems and methods for differentiating an eddy current sensor signal induced by a substrate from a signal induced by a thin-film on the substrate.
2. Description of the Related Art
An ECS (eddy current sensor) detects a conductive material by generating and projecting an electromagnetic field (EMF) and detecting a change in the EMF when the conductive material (e.g., a wire, a conductive film, etc.) is placed into the space where the EMF is generated. When the conductive material is placed in the EMF, an eddy current is induced in the conductive material to compensate for the electrical field penetrating into the volume of the conductive material. The eddy current generates its own EMF, which interacts with the primary EMF resulting in a compensative change of the EMF. The change in the EMF is detected by the ECS. Through various calibration techniques an ECS a known distance from a conductor can determine certain aspects of the conductor from the effect the conductor exerts on the EMF. The amplitude of the EMF change depends on the resistance of the conductive material and the proximity of the conductive material to the ECS. By making constant some variables combined with various calibration techniques, this property of the ECS can be used to determine various aspects of the properties of the conductive material as well as its proximity to the ECS,
By way of example, an ECS produces an EMF consisting of a 1 megahertz (MHz) signal. A conductive film (e.g., copper, aluminum, etc.) on a silicon substrate is passed through the EMF. The EMF induces an eddy current into the conductive film and the induced eddy current interferes with the EMF. The ECS detects an ECS signal that is a result of the interference caused by the eddy current.
FIG. 1 shows a typical ECS 110. A substrate 120 has a conductive film 130 or layer thereon and an EMF 112 emitted from the ECS 110 (not drawn to scale). Typically, the EMF 112 is considered to effectively penetrate through a conductor a depth quantity referred to as xe2x80x9ca skin depth.xe2x80x9d A skin depth is the distance into a target (e.g., a conductor), which an EMF wave will decay to about 1/e (about 37%) of the initial value of the EMF wave. Skin depth is a function of the frequency of the EMF 112 and the conductor material type and other factors. If the conductor is the conductive film 130 and the conductive film is copper, the skin depth is about 220,000 angstrom, at 1 MHz. If the copper film 130 is thinner than the skin depth (e.g. about 5000 angstrom), then the EMF 112 will induce an eddy current in both the copper film 130 and the substrate 120.
The resulting signal that is detected by the ECS 110 includes components attributable to both an eddy current induced in the substrate 120 and an eddy current induced in the copper film 130. However, even if the conductor 130 were thicker than skin depth, the EMF does not actually stop penetrating at skin depth as at least part (e.g., about 37%) of the EMF penetrates further beyond the conductor 130 (e.g., into the substrate 120 and the environment beyond the substrate 120). In the present example, where the conductor 130 is thinner than skin depth, a large portion of the EMF penetrates into and even through the substrate 120 a penetration distance 114.
However, because the substrate 120 offers significant resistivity, a very small eddy current is induced in the substrate 120. As a result, the majority (e.g., about 90-95%) of the detected ECS signal is due to the eddy current induced into the conductor 130. Only about 5-10% of the detected ECS signal is due to the eddy current induced in the substrate 120.
Unfortunately, if the substrate 120 is a silicon substrate, the resistivity of the silicon substrate 120 can vary from edge to center due to the varying physical characteristics (e.g., crystalline structure, dopant concentration, and other physical characteristics) of the crystal from which the substrate was cut. Because the resistivity varies, the eddy current in the silicon substrate 120 can also vary a proportional amount between the center and the edge of the substrate 120.
A typical silicon substrate is identified as having an xe2x80x9caverage resistivityxe2x80x9d value. The average resistivity value indicates that it is possible for the resistivity at the edge of the substrate 120 to be half the resistivity at the center of the substrate 120, resulting in a 100% or more variation in resistivity. By way of example, if a wafer can be labeled as having an average resistivity of 1.0 ohm/cm. A resistivity of 1.0 ohm/cm could allow a resistivity of 0.5 ohm/cm on the edge of the wafer and a resistivity of 1.5 ohm/cm or more at the center of the wafer, resulting in a variation of 300% or more between the edge and the center. Silicon substrates can also be labeled with a range of resistivity (e.g., 0.008-0.020 ohm/cm) indicating that the resistivity anywhere on the wafer will fall within the stated range. A range of 0.008-0.020 ohm/cm allows for a 250% variation in resistivity. Therefore, even if only about 5-10% of the detected ECS signal is due to the eddy current induced in the substrate 120, the 5-10% can vary widely. By way of example, between about 2% and about 6% or between about 4% and about 10%.
This variation in the detected ECS signal due to the substrate 120 makes it difficult to accurately detect the component of the detected ECS signal that is attributable to the thin conductive film 130. What is needed is a system and method for minimizing or eliminating the component of detected ECS signal resulting from the eddy current induced in the substrate.
Broadly speaking, the present invention fills these needs by providing an improved system and method of measuring an ECS signal. It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, computer readable media, or a device. Several inventive embodiments of the present invention are described below.
One embodiment includes a method for determining a component of an eddy current sensor (ECS) signal attributable to a substrate. The method includes placing a substrate in a first position relative to an ECS at a first distance from the ECS. The substrate can include a conductive film on a first surface of the substrate. A first ECS signal can be detected with the substrate in the first position. The substrate can then be inverted relative to the ECS such that the substrate is in a second position relative to the ECS at a second distance from the ECS. The second distance is equal to the first distance less about a thickness of the substrate. A second ECS signal is detected with the substrate in the second position. A difference signal is determined. The difference signal is equal to a difference between a first signal level on a calibration graph for the ECS and the second signal level. The second signal level being shifted a distance about equal to the thickness of the substrate. A first substrate component of the first ECS signal is calculated. The first substrate component of the first ECS signal is equal to a product of the first distance and the difference signal, divided by the thickness of the substrate.
The conductive film has a thickness of between about 10 and about 20,000 angstroms. The conductive film is a film residue.
The ECS can be aligned with a first point in both the first position and the second position, the first point being on the first surface of the substrate.
The conductive film is juxtaposed between the substrate and the ECS in the first position.
Inverting the substrate can include moving the ECS.
Inverting the substrate can include moving the substrate.
Inverting the substrate can include adjusting the substrate an amount equal to about a thickness of the conductive film.
The method can also include calculating a second substrate component of the second ECS signal. The second substrate component of the second ECS signal is equal to a product of the second distance and the difference signal, divided by the thickness of the substrate.
The method can also include calculating a component of the first ECS signal attributable to the conductive film. The component of the first ECS signal attributable to the conductive film is equal to a difference between the first ECS signal and the first substrate component of the first ECS signal. A thickness of the conductive film can also be determined.
Another embodiment includes a method for mapping a resistivity of a substrate. The method includes determining a component of the eddy current sensor (ECS) signal attributable to the substrate relative to a first point and a second point on the surface of the substrate. A first resistivity is calculated for the first point and a second resistivity is calculated for the second point. A resistivity curve can be extrapolated from the resistivity at the first point and the second point.
Another embodiment includes a system for determining a component of an eddy current sensor (ECS) signal attributable to a substrate. The system includes an ECS oriented toward a substrate. The substrate is in a first position relative to the ECS at a first distance from the ECS. The substrate includes a conductive film on a first surface of the substrate. A substrate inverter is also included. The substrate inverter is capable of inverting the substrate relative to the ECS such that the substrate is in a second position relative to the ECS at a second distance from the ECS. The second distance is equal to the first distance less about a thickness of the substrate. A control system is coupled to the ECS. The control system includes logic that detects a first ECS signal with the substrate in the first position and logic that detects a second ECS signal with the substrate in the second position. The control system also includes logic that determines a difference signal equal to a difference between a first signal level on a calibration graph for the ECS and the second signal level. The second signal level being shifted a distance about equal to the thickness of the substrate. The control system further includes logic that calculates a first substrate component of the first ECS signal equal to a product of the first distance and the difference signal, divided by the thickness of the substrate.
The system can also include a stage for supporting the substrate in the first position and the second position. The stage can be adjustable to compensate for a thickness of the conductive film.
The substrate inverter can include an end effector that can invert the substrate. The substrate inverter includes an actuator that moves the ECS.
The ECS can also include a first ECS and a second ECS and wherein the substrate is in the first position relative to the first ECS and the substrate is in the second position relative to the second ECS. The first ECS and the second ECS are substantially aligned.
The first distance can be substantially equal to the second distance.
The first ECS and the second ECS are operated about 180 degrees out of phase.
The present invention provides for more accurate measurement and detection of conductive films on a substrate and by accurately determining the resistivity of the substrate.
Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.